So far as one of the methods used for detecting the presence or absence of objects to be detected, the quantities and positions thereof, there has been a method for irradiating ultraviolet rays to an object to be detected, to which is glued a fluorescent substance or to which is applied or added a fluorescent agent for which the luminous emission from the fluorescent substance or agent is detected by a photosensor. However, the intensity of luminous emission from a fluorescent substance is fundamentally very weak, such that in order to improve the detection rate, a sensor, an ultraviolet-ray source and a work must be surrounded by light-shielding plates or boxes so as to eliminate adverse effects due to external light rays or the like. Consequently, there exists the defect that special boxes or the like must be provided, and thus, the range of utilization is considerably limited. Therefore, in order to improve the above-mentioned method, there has been proposed a method for disposing an optical filter with a narrow transmission wavelength band in front of a photosensor so as to selectively detect only the luminous emission from a fluorescent substance. However, according to this method, the luminous emission from the fluorescent substance for which the intensity is fundamentally considerably weak, is received through a filter with a low degree of transmissivity so that the output signal from the photosensor is further decreased in strength. As a consequence, there arises the defect that not only does the amplifier circuit in the succeeding stage become complex in construction, but also a reduction in the S/N ratio results. In addition, in order to process the above-mentioned signal, an expensive device of complex construction is needed. As described above, there has not been available so far a method capable of detecting a fluorescent substance in an inexpensive and simple manner, and consequently, visual inspection is presently employed in practice. However, according to the latter method, a person must continue a simple task for a long period of time and furthermore, must continuously observe the weak light in a dark place. Thus, there arise the problems of the production efficiency being considerably low; resultant individual differences; and the method not being safeguarded against health hazards.
Furthermore, in order to detect the failure and state of adhesive application in the adhesive application process for devices such as collators, box-making machines and book-binding machines or the like, a photosensor has been utilized in such a way that light is projected on an applied paste and the light reflected therefrom or transmitted therethrough is intercepted by the photosensor, whereby the quantity of applied paste or whether or not the applied paste exists, is detected in response to the variations in the light-reception signals output from the photosensor. However, according to this method, whether or not a paste exits is detected in response to the intensity of the reflected or transmitted light which is received, requiring that a light beam with a predetermined intensity must be projected at an object to be detected, while maintaining a predetermined distance and angular relationship between an object to be detected and the photosensor. In addition, objects to be detected or the color and composition of an object to be compared must be maintained uniform. For instance, in the case of collators, in most cases an adhesive is applied at the width of 1 mm and the light beam with a predetermined intensity must be projected at the applied paste which is fine in shape and small in quantity so that extremely delicate setting conditions are required. As a result, the above-mentioned detection method has a defect that the degree of operability is considerably worse. There exists the further problem that sheets of paper or film which are to be applied with an adhesive have been already printed, so that decolorization inevitably results.
In view of the above, there has been proposed a method which takes into consideration the water content in a paste or adhesive so that the detection of the quantity of applied adhesive or the detection whether or not there exists an applied adhesive is made in response to the difference in electrostatic capacitance between sheets of paper or film due to the difference in water content therein. However, when there exists a piece of metal in the vicinity of an electrostatic-capacitance sensor, this measurement becomes impossible. Furthermore, in order to measure the difference in extremely low electrostatic capacitance, the fine position arrangement of a probe must be made. This method has a shortcoming that the measurement is easily and adversely affected by vibration of an object to be measured. Thus, this method has the defects of worsened operability and adversely affected production to a remarkable degree.
As described above, the conventional detection methods are unsatisfactory in practice so that at present, an inspector makes a visual or sampling inspection or he/she makes a total or 100% inspection in another step so as to detect the failure of adhesive application. However, it is physically impossible to completely eliminate the objects which must be rejected by the visual or sampling inspection, and differences in the results of inspection between individual inspectors occur. Furthermore, however highly improved the performance of a device is, the workers have not been completely freed from the inspection processes. And moreover, even when the total or 100% inspection is made by a worker in another step, he/she must continue a simple task for a long period of time, so that there arises the problem that the production efficiency is considerably degraded.
In addition, in order to improve the qualities of products, the quantity of applied adhesive, and whether or not there exists an applied adhesive, must be accurately and properly detected so that the quantity of applied adhesive is controlled to a predetermined value, whereby the products to be rejected can be fundamentally eliminated.